Although thermal angioplasty is being used in patients, the biological responses to varying catheter-tip temperatures in small arteries is unknown. Therefore, we studied a canine model consisting of autologous fat, Gelfoam implants into small femoral arteries, followed by prototype catalytic thermal-tip catheter angioplasty. We found that irreversible vasospasm frequently occurs during thermal angioplasty, especially at higher catheter- tip temperatures with prolonged catheter-tissue contact and in smaller vessels which are closely matched to the catheter size, particularly in low flow states, such as severe stenoses or total occlusions.